Polyimide films have good mechanical properties and other good characteristics such as heat resistance, chemical resistance, and electrical insulation properties and are therefore widely used as various films for optical waveguides and electronic devices such as interlayer dielectric films for semiconductors, buffer coatings, substrates for flexible printed circuit, and alignment films for liquid-crystal displays.
Glass is widely used as substrates for liquid crystal displays, organic electro luminescence displays, organic thin-film transistors, etc. until now. However, with the trend toward lightweight, flexible display, flexible substrates made of plastics such as PEN (polyethylene naphthalate) and PES (polyethersulfone) are being developed. The flexible substrate should have high transparency, low thermal expansion, and high glass transition temperature. In particular, the light transmittance of a film of 10 to 30 μm in thickness must be 80% or more; in order to prevent improper arrangement of display pixels and wiring deviation on a substrate due to expansion or contraction of the substrate, the thermal expansion coefficient must be 20 ppm/° C. or less in the range of 100 to 300° C.; and glass transition temperature must be 350° C. or higher.
It is known that the introduction of a highly flexible monomer or a fluoro group into the polyimide skeleton is effective to obtain a highly transparent polyimide film. For example, a totally fluorinated polyimide obtained from 2,2-bis(3,4-carboxyphenyl)hexafluoropropanoic dianhydride (a fluorinated acid dianhydride) and 2,2′-bis(trifluoromethyl)benzidine (a fluorinated diamine) is known. A film having the thickness of 20 μm obtained from the aforementioned totally fluorinated polyimide exhibits a light transmittance as high as 85% at wavelength of 400 nm. However, the coefficient of thermal expansion of the film is as high as 48 ppm/° C.